It's interesting though because if you ask, you know, a five year old, they're not going to tell you any such thing. With the exception of cases where a kid is noticeably behind or nonverbal, something about the way young kids learn language appears to make it way easier compared to what adults are often doing in order to adopt a second or third language.

It's something I want to research and understand better, but one concept I recall exploring (offhand, am unsure how much this is backed by research in the specifics) is to do with strength of associations. Like if we consider a little baby learning the word "mama" (or its various counterparts in different languages), they aren't being handed flashcards and trying to memorize "mama" like it's a textbook word that has no relevance to them until they decide to apply it in real life. They are learning to associate "mama" with all kinds of sensory information, physical needs, emotional needs, memories of events (hopefully mostly fond ones), and so on.

So I suspect where some of the friction lies is in adults trying to learn language words like they are learning historical facts; as distant snippets of information that are supposed to be memorized exactly as they are, so that they can be produced on request to prove their knowledge for a test. Whereas if they are learning them more in association the language's culture, with interaction in that culture, or more generally, learning it in strong association with things like sensory information and others listed above, then it will feel like it's being learned more naturally. But the question remains of how to do this in a world that is increasingly abstracted out to digital, with lots of people who are interested in learning additional languages but have no organic avenue to immersion.

Use Anki! It's yet another cue card app but it does varied spaced repetition so it codifies educational best practices built in. Sorry for sounding like an ad read but I swear by it lol